Many processes for producing a dispersion of rutile titanium oxide particles are already conventionally known. The most general process is the one in which a titanium salt is hydrolyzed to prepare slurry of hydrous titanium oxide, and the slurry is peptized with an acid or an alkali to obtain a dispersion. However, the particles of titanium oxide in the dispersion of rutile titanium oxide obtained in this conventionally known process have a D90 of 40 nm or more, and therefore they cannot be used in such an application of a resin molding or a coating agent for hard coating where the particles are required to have a high transparency.
Recently, some processes for producing a dispersion of fine particles of rutile titanium oxide are proposed. For example, a process is proposed in which titanium tetrachloride and hydrochloric acid are mixed in water having a temperature of 65 to 90° C., and the mixture is heated to a temperature of 65° C. to a boiling point of the resulting mixture to hydrolyze the titanium tetrachloride, thereby a dispersion of particles of rutile titanium oxide is obtained (JP 2006-335619A). According to this process, a dispersion of fine particles of rutile titanium oxide having a BET specific surface area within a range of 50 to 300 m2/g can be obtained, but the process has a defect in which the particles of titanium oxide have a low crystallinity. In general, the higher the crystallinity of the particles, the more effective the exhibition of various physical properties of the rutile titanium oxide such as UV shielding property, photocatalytic activity, and a refractive index, and hence the higher crystallinity is preferable. The increase of the crystallinity can be confirmed by increase of peak intensity of a powder X-ray diffraction without being accompanied by particle growth.
Furthermore, it is believed that the particles of rutile titanium oxide obtained by thermal hydrolysis of titanium tetrachloride are not increased in crystallinity because an amorphous titanium compound is intermixed therein, and that there is a volatile component such as water which is contained in or adheres to the particles. For comparison of the amounts of the amorphous titanium compounds, variations in weight loss on heating may be compared. It can be said the smaller the weight loss on heating, the higher the crystallinity.
It is traditionally known that the crystallinity of the rutile titanium oxide can be increased by calcining it, but when the oxide is calcined, the particles grow at the same time, and therefore the crystallinity cannot be increased while the particles remain fine. To be highly crystalline is traded off against to be fine in particle size.
As another process for producing a dispersion of fine particles of rutile titanium oxide, for example, a process is proposed in which fine particles of rutile titanium oxide which have been previously produced are dispersed in water (JP 07-232925A). According to this process, however, the particles of titanium oxide dispersed in water have a minimum average particle size of 70 nm.